Creating hybrids may save Asia’s tigers, say scientists

An international team of researchers has proposed lowering the number of tiger subspecies from today’s widely accepted nine to just two, saying efforts at conserving the endangered animals will benefit as a result.

“Only the Sunda tiger – on the [Indonesian] islands of Sumatra, Java and Bali – and the continental tiger are clearly distinct [subspecies],” said Andreas Wilting, lead author of the study that led to the proposal.

The research team analysed variation among all nine putative tiger subspecies – three already extinct and a fourth probably surviving only in captivity, they said – using data on the structure of more than 200 tiger skulls, the colouring and pattern of stripes and spots on more than 100 tiger hides, as well as molecular and ecological data taking climate, habitat and prey into account.

“Our analyses revealed little variation and large overlaps in each trait,” the researchers write in the US journal Science Advances, arguing for subspecies recognition of only the Sunda tiger (Panthera tigris sondaica) and the continental tiger (Panthera tigris tigris), the latter of which consists of two “conservation management units”: northern, including the Amur tigers of Siberia; and southern.

“The previous taxonomy of tigers is no longer tenable,” declared Wilting, head of the junior research group on the biodiversity and biogeography of Southeast Asia at the Leibniz Institute for Zoo and Wildlife Research (IZW) in Berlin.

He said a global consensus on just two subspecies and three management units would allow conservation management programmes such as captive breeding to be implemented “more flexibly and effectively.”

Today only 3,200 to 3,600 tigers, the largest of all wild cats, are estimated to roam the forests of Asia, occupying just 7 percent of their historical range. The number was about 100,000 a century ago. And the population continues to shrink.

According to Wilting, basing conservation strategies on numerous subspecies for which there is little or no scientific support may actually hinder the tiger’s survival by preventing large-scale cooperative conservation management programmes which mix tigers from different countries.

What is more, he pointed out, “hybrid” tigers of supposedly different subspecies are being excluded from many conservation campaigns, greatly reducing the number of available animals, unnecessarily restricting gene pools and thus impeding the success of conservation measures.

As an example, Wilting cited the difficulty in conserving the severely decimated populations of South Chinese and Indochinese tigers, both currently regarded as subspecies.

Grouping them together with Malayan and Bengal tigers as “southern continental tigers” would make conservation easier, he said.

“One conclusion of the study is that the threat to a particular tiger subspecies sinks when more tigers belong to it,” noted Volker Homes, director of the species conservation section of the World Wide Fund for Nature (WWF) Germany.

Tiger-range countries must not, however, shift their responsibility for protecting the animals to their neighbours simply because genetically similar tigers live there, he said.

Related Stories

Wilting said the finding of extremely low genetic diversity among tigers supported the theory that an eruption of the super-volcano Toba in northern Sumatra about 73,000 years ago during the Late Pleistocene era had led to mass extinction of the cats.

“Tigers may have only survived in a single refugium [outside the region of ash-cloud fallouts] in southern China and spread out from there,” Wilting said. “These animals could have been the progenitors of all modern tigers.”

Poaching and habitant loss due to illegal logging pose the greatest threats to tigers today. There is a demand for tiger products, particularly for traditional Chinese medicine. Homes said tiger conservation was well developed in some countries, including Nepal, India and Russia, where “the number of tigers is rising”.

On the other hand, Indonesia and Malaysia don’t even count their tiger populations, he said, adding, “If you don’t know how many tigers you’ve got, you don’t know how fast the populations are declining.”

According to Homes, the animals have largely died out in Cambodia, Laos and Vietnam, but Thailand and Burma have the potential to quickly boost their tiger populations.

India reported in January that the number of tigers there had risen by almost a third over the previous three years: from 1,706 in 2011 to 2,226 in 2014. About 70 percent of the world’s wild tigers live in India.